1. Field of the Invention
The field of the invention is that of very-long-distance digital transmission (several thousands of kilometers) by optical fiber in systems using on-line optical amplification
One of the main factors limiting the bit rate in very-long-distance systems such as these is the distortion induced by the transmission fiber.
For, while these distortions may be more or less overlooked in standard fiber-optic systems (setting up links over distances in the range of some humdreds of kilometers), they have, on the contrary, very disturbing effects on long-distance transmission systems.
The invention relates to a system of transmission on an optic fiber line enabling compensation, at reception, for this distortion induced by the transmission line, in the case of transmission lines having a length of at least a thousand kilometers, such as those used for trans-ocean links.
The distortion contributed by the transmission fiber arises out of the combined existence of two phenomena that occur in the monomode fibers: chromatic dispersion and non-linear effects.
The first phenomenon is that of chromatic dispersion. This phenomenon results from the frequency dependency of the refractive index of silica. It entails different propagation times depending on the operating wavelength. In general, chromatic dispersion tends to widen the pulses of the digital trains and, hence, to create inter-symbol interferences.
In the commonly-used fibers, the chromatic dispersion is zero around 1.3 .mu.m and takes a positive value of about 17 ps/nm/km around 1.55 .mu.m. It is also possible to use dispersion-shifted fibers which are designed to have zero chromatic dispersion in the region of 1.55 .mu.m.
Very-long-distance transmission systems (covering several thousands of kilometers) work at 1.55 .mu.m. The excessive value of the chromatic dispersion of the commonly used fibers at this wavelength rules out their use. Hence, dispersion-shifted fibers will be used in these cases.
It must be noted that the effect of distortion by chromatic dispersion depends greatly on the spectral components of the pulses: if a pulse shows variations in optical phase that are positive at its start and negative at its end, it will be greatly widened by a positive chromatic dispersion. The converse is true for negative dispersions.
The second phenomenon relates to the non-linear effects. The most important non-linear effect in a fiber is the Kerr effect. This effect, which is described for example in the document by K. W. Blow and N. J. Doran, "Non-linear Effects in Optical Fibers and Fiber Devices" (IEEE Proceedings, June 1987, pp. 138-144) reflects a linear dependency of the refractive index of silica with respect to the optical power.
The non-linear effect is very low in the usual fields of operation of the optical systems (distance smaller than 400 km and power below about 10 mW), but becomes non-negligible for very high power values (of the order of 1 W) or for very large propagation distances at reasonable levels of power (some thousands of kilometers in a periodic amplification system).
When there is no chromatic dispersion, the Kerr effect induces a self-phase modulation of the optical pulse: the instantaneous frequency diminishes at the start of the pulse and then increases at its end, proportionally to the derivative of the optical power. This induces a widening of the spectrum and a spectral composition that fosters a substantial widening for negative chromatic dispersions.
The distortion provided by the transmission fiber should be considered as the combination of the chromatic dispersion (the first phenomenon) and of the non-linear effects (the second phenomenon).
The combination of these two effects may be described by a non-linear equation with partial derivatives of distance and time, known as Schrodinger's non-linear equation, the resolving of which is discussed notably in the work by G. Agrawal, "Non-Linear Fiber Optics", Academic Press.
The numerical resolution of this equation shows that there are two forms of behavior which are qualitatively very different depending on the sign of the chromatic dispersion (D):
* First case: D&gt;0. In this case, phenomena of instability of modulation are observed. The pulses "burst" into very short pulses at the end of 1000 to 2000 km and the optical spectrum widens considerably: this may give rise to problems related to the optical passband. PA1 * Second case: D&lt;0. There is no instability of modulation and the pulses keep a certain degree of integrity while the spectrum widens quasi-monotonically during the propagation, while keeping reasonable widths. However, the pulses widen greatly temporally, thus creating inter-symbol interferences. These interferences become very troublesome for example, as soon as the chromatic dispersion goes beyond 0.05 ps/nm/km in terms of absolute value for bit rates of 5 Gbits/s on distances of 6000 to 8000 km.
The most efficient case is naturally the second one, that of a negative chromatic dispersion. However, to make the very-long-distance systems work in negative dispersion mode, the values of chromatic dispersion of the fibers used must obligatorily be very low.
2. Description of the Prior Art
Any method of compensation for the two phenomena that are the cause of the distortion in the fiber (chromatic dispersion and non-linear effects) is therefore of great value since it can be used to overcome the drawback of the low values imposed on the chromatic dispersion. Indeed, by using a method of compensation for the distortion provided by the transmission fiber, it is possible to envisage two strategies.
In a first strategy, for given characteristics of negative chromatic dispersion of the transmission fiber, the compensation can be used to increase the product: line bit rate * range of the link.
In a second strategy, for a fixed line bit rate and a fixed range, the compensation makes it possible to use transmission line fibers having less stringent constraints as regards the characteristics of chromatic dispersion. These fibers are easier to manufacture on an industrial scale and to sort out for the setting up of an underwater link for example.
There are known methods of compensation for the distortion introduced by a fiber-optic transmission line.
A first known method, described for example by A. H. Gnauck et al in the article "Dispersion Penalty Reduction Using an Optical Modulator with Adjustable Chirp" OFC 1991, post-deadline paper No. 17, consists of the use, at transmission, of an optical amplitude modulator having adjustable chirp (or variation in instantaneous optical frequency).
This first method has been proposed solely in order to provide partial compensation for the penalties of chromatic dispersion on standard optical fibers in short-distance transmission systems (of some hundreds of kilometers).
Indeed, it has not been sought, in this method, to compensate for the non-linear effects, which are completely negligible in this case, but solely to compensate for chromatic dispersion.
Besides, this first known method specifically relates to standard optical fibers (with zero chromatic dispersion around 1.3 .mu.m). Now, in very-long-distance transmission systems (of several thousands of kilometers), the excessive value of the chromatic dispersion of the commonly used fibers rules out their use, and then it is generally dispersion-shifted fibers (namely fibers with zero chromatic dispersion around 1.55 .mu.m) that are used.
Finally, the compensation takes place at the transmitter station and hence by anticipation, and not at reception, on the actually disturbed signal.
It can therefore be clearly seen that this first known method is not suited to very-long-distance transmission systems and does not relate to a system of compensation for distortions at reception.
A second known method of compensation, described for example by T. Koyama et al. in the article, "Compensation for Non-Linear Pulse Distortion in Optical Fiber by Employing Prechirp Technique", ECOC 91., WeC7-2 p. 469, consists of compensation, at transmission, for the self-phase modulation generated by the non-linear effects.
To this end, the pulses are made to undergo a continuous scanning of the optical frequency, for the duration of a bit, by over-modulation of the sending laser.
What is used in this case is the fact that the optical frequency of a semiconductor laser is a function of its current. However, only certain structures of lasers can thus display high efficiency of frequency modulation, i.e. a major variation of this optical frequency, for a small variation of the control current and hence of the optical power.
This second known method has the drawback of requiring a particular over-modulation, which is synchronous with the useful digital train, on the sending laser used. Consequently, only certain types of lasers, those capable of providing a major variation of the optical frequency for a low variation of the control current, may be used in this second method.
Furthermore, this method can be applied only for a particular binary coding, namely the on-line RZ coding.
Furthermore, just as in the first method described, the compensation takes place at transmission and not at reception.
Finally, this method enables compensation solely for non-linear effects.
There are other known methods of compensation that work for the correction, at reception, of the distortion due to the combination of high positive chromatic dispersions and non-linear effects (in particular the chirp of the laser, this "chirp" of the laser being a parasitic effect that consists of a variation of the instantaneous optical frequency, during a pulse, especially at the beginning and end of this pulse).
These other methods use different structures (a Fabry-Perot filter for example) which have the drawback of correcting the distortions only very partially.
In particular, the non-linear effects are not compensated for by these known equalizers or filters. Now, the change from a transmission line with a length of some hundreds of kilometers to a line with a length of over a thousand kilometers leads to a situation where it is not longer possible to overlook the non-linear effects.
Indeed, these non-linear effects become as important, if not more important, than chromatic dispersion. Consequently, a specific solution has to be found in order to resolve this new problem.
The invention is aimed notably at overcoming these different drawbacks of the prior art.
More specifically, it is an aim of the invention to provide a system capable of compensating for the distortion induced by a transmission line with a length of at least a thousand kilometers on a monomode optical fiber with negative chromatic dispersion.
Another aim of the invention is to provide a system such as this providing compensation for the combination of the two phenomena induced by the transmission fiber, namely the negative chromatic dispersion and the non-linear effects, and not solely for either one of these phenomena.
The invention is also aimed at providing a system such as this, capable of being implemented irrespectively of the type of laser used at transmission.
Another aim of the invention is to provide a system such as this that is simple to implement and reliable and that costs little.